Semiconductor (SC) devices are often encapsulated in molded plastic. The molded plastic surrounds and protects the semiconductor die, supports the bonding wires and external leads and imparts ruggedness and shock resistance to the device. Plastic packaged devices are widely used. FIG. 1 shows a simplified schematic cross-sectional view through prior art molded plastic package 20 containing semiconductor (SC) die 22. SC die 22 is conveniently but not essentially mounted on heatsink 23. Metal contact regions 24-1, 24-2 (collectively 24) are provided on SC die 22 to which external leads 26-1, 26-2 (collectively 26) are coupled by wire bonds or other means 25-1, 25-2 (collectively 25). Plastic encapsulant 27 is molded around SC die 22, wire bonds 25 and inner portions 28-1, 28-2 (collectively 28) of external leads 26, so that, in this example, lower surface 21 of heatsink 23 remains exposed on the lower face of package 20, but having surface 21 exposed is not essential. While plastic encapsulation, such as is illustrated in FIG. 1 and equivalents, is widely used, it suffers from a number of disadvantages and limitations well known in the art. Among these are that plastic material 27 surrounding SC die 22 and leads 25 and 28 has a significantly higher dielectric constant ε and loss tangent δ than does air or vacuum. For example, commonly used plastic encapsulants for semiconductor devices often have relative dielectric constants ε in the range 3.5 to 5.0 and loss tangents δ in the range 0.005 to 0.015 at the operating frequencies of interest. These are sufficient to result in significant degradation of performance, especially at high frequencies and high voltages, from electrical cross-talk through the plastic encapsulation between the various die metal regions, bonding wires and other leads, due to fringing electric field 29 extending into surrounding plastic encapsulant 27. The capacitive coupling and loss associated with fringing electric field 29 increase as the dielectric constant ε and loss tangent δ of encapsulant 27 increase.
In the prior art, the capacitive coupling and loss associated with this fringing electric field extending outside of the SC die has been mitigated or avoided by, for example: (i) using a Faraday shield (not shown) over the die and/or wire bonds, and/or (ii) using hollow ceramic and/or metal packages that provide an air or vacuum space above the die surface and around the wire bonds and inner package leads. A Faraday shield constrains the fringing fields but at the cost of additional die complexity due to the additional metal and masking layers required. A vacuum or airspace package is illustrated in FIG. 2, which shows hollow package 30 having air or vacuum space 37 surrounding die 32. Die 32 is mounted on, for example, metal, ceramic or plastic base 33 to which are attached external leads 36-1, 36-2 (collectively 36). Wire bonds or other connections 35-1, 35-2 (collectively 35) couple bonding pads 34-1, 34-2 (collectively 34) on die 32 to inner portions 38-1, 38-2 (collectively 38) of package leads 36-1, 36-2 (collectively 36). Cap 31 is placed over substrate 34, die 32, wire bonds or other connections 35 and inner portions 38 of package leads 36. Having air or vacuum space 37 around die 32 and leads 35, 38 means that fringing electric field 39 is not in contact with any encapsulant. Therefore there is no increase in coupling capacitance and/or loss caused by a plastic encapsulant in contact with the die surface and wire bonds and/or inner leads. The dielectric constant ε0 and loss tangent δo of air or vacuum are low and so cross-talk and dielectric loss are minimized. However, such air or vacuum cavity packages are significantly more expensive and often not as rugged as plastic encapsulation. Wire bonds or other connections 35 can become detached if the finished device is subjected to large acceleration forces.
Thus, there continues to be a need for improved semiconductor devices and methods that provide plastic encapsulated devices with reduced cross-talk and loss. Accordingly, it is desirable to provide improved semiconductor devices with plastic encapsulation having lower dielectric constant ε and/or the loss tangent δ material in contact with some or all of the die surface, die leads and/or bonding wires. In addition, it is desirable that the improved plastic encapsulation materials, structures and methods allow a substantially solid structure to be formed surrounding the semiconductor die, die leads and bonding wires so as to provide a mechanically rugged package. It is further desirable that the improved devices be achieved using fabrication technology already available on or easily added to a typical semiconductor device manufacturing line so that only minor modification of the manufacturing process is required. Other desirable features and characteristics of the invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.